This invention relates to an arrangement for rotating a diffraction grating in a spectral device within the scanning angle, wherein a light beam impinges on the grating at a large angle of approximately 60.degree..
Known sine and cosecant control devices, suitable for common types of diffraction gratings, i.e., of gratings used at small angles of impingement of light beams, cannot be applied for gratings adapted to work at large angles of impingement with respect to light beams, for kinematic reasons. The known arrangement of a sine control device includes an arm fixed at one end to the rotatable table which supports the diffraction grating. The arm slides by means of a spherical or cylindrical cam, fixed on its other end, along a straight strip. The strip is fixed to an element moving along a straight line in a direction perpendicular to the movement of this element. This element can be for instance a nut on a motion screw. The straight strip is fixed on the nut to extend in a direction perpendicular to its longitudinal axis. The motion screw is situated so that its axis does not point towards the center of the rotatable table and is also not parallel with the direction of light beams impinging on the diffraction grating. This arrangement provides a linear distribution of the spectrum according to wave lengths .lambda..
The arrangement of a known cosecant control device includes an arm fixed at one end to the rotatable table which supports a diffraction grating. The other end of the arm is provided with a planar strip. The plane of the strip points to the center of rotation of the rotatable table. The arm rests, by means of this planar strip, against a spherical or cylindrical cam fixed to an element moving along a line pointing equally to the center of rotation of the table and parallel with the direction of light beams impinging on the diffraction grating. This element can be for instance a nut driven by a screw bolt. The cam in this case is fixed to extend in the direction of the longitudinal axis of the nut. This arrangement provides a distribution of the spectrum according to wave numbers (cm.sup..sup.-1).
These arrangements are for gratings of the echelle type substituted by different kinds of control devices. They have the drawback, however, that they do not provide linearly distributed spectra behind the exit slot of the spectral device, either in wave numbers or in wave lengths.